This invention relates generally to radio communication utilizing radiating transmission lines and more particularly to an improved method and apparatus for utilizing radiating transmission lines to provide multiple channels of mobile-to-mobile radio communication in tunnels, mines, building and other confined spaces.
The near impossibility of wireless communication by conventional broadcast radio techniques in tunnels, mines and buildings has been known from the beginning of radio communication. Due to well-known physical principles, an electromagnetic wave will not propagate for any appreciable distance in a confined space having lossy, discontinuous or irregular boundaries. For example, it is not only impossible to broadcast a radio frequency signal for more than a limited distance into a mine or tunnel from an outside point but it is also impossible for two mobile radio transceivers both located in a mine or tunnel to communicate with each other for more than a limited distance. Similarly, buildings constructed of steel and concrete define spaces in and into which conventional radio communication is difficult and unreliable. Even natural or man-made hills and valleys in the surface of the earth define areas which present difficulties in communicating by conventional broadcast or mobile radio techniques.
An early attempt to approach the advantages of radio communication in tunnels, mines and buildings was based on low frequency inductive coupling between mobile transceivers and a conductive wire extending throughout the space to be served. A low frequency electrical signal can be transmitted for a great distance along a conductive wire before resistive and inductive effects attenuate it to a value too low to be useful, whereas a high frequency signal will be attenuated in a relatively short distance, as is well known in the telephone art. This difference is largely due to the fact that high frequency energy will be radiated from the wire to a far greater extent than low frequency energy. Thus, at low frequencies, very close physical proximity is required between the conductive line and the transceivers to provide reliable inductive coupling of signals therebetween. The requirement for close physical proximity between the line and the transceiver makes the low frequency inductive coupling approach substantially equivalent to direct wire connections of the telephone type between mobile units with all of the disadvantages thereof.
Another approach has been to provide a plurality of fixed transceivers distributed throughout the tunnel, mine or other area to be served with their antennas radiating to adjacent limited areas and with all of the fixed transceivers interconnected by telephone lines. With proper control circuitry this system can provide satisfactory coupling to and from mobile transceivers. A radio signal from a first mobile transceiver will activate the receiver of the fixed transceiver covering the area in which the first mobile transceiver is located and the demodulated audio signal produced by such receiver will be transmitted over the telephone lines to activate the transmitters of the other fixed transceivers. Thus, the radio frequency signal from the transmitter of the other fixed transceiver will reach a second mobile transceiver in the tunnel and two-way communication can be established according to conventional radio techniques.
However, it is impossible to provide complete coverage of the mine, tunnel or other space without overlapping of the coverage of one or more of the fixed transceivers. If the coverage is not complete, then there will be dead areas that cannot be reached by radio signals and from which no radio signals can be received. If there is overlapping coverage by two or more fixed transceivers, then radio signals in the overlapping areas will interfere with each other in their effect on the control system and will make communication unreliable to and from the area in which overlapping coverage is present.
In the United States, improved coverage with reduced overlapping has been obtained in the above described system of fixed transceivers interconnected by telephone lines through the substitution of radiating transmission lines for the antennas of the transceivers. Thus, the antenna connections of each transceiver have been connected to the midpoint of a length of conventional high frequency balanced twin lead transmission line, for example. Radio frequency energy will propagate along such a transmission line for a greater distance than along a single wire conductor since radio frequency energy will be radiated from the balanced transmission line more evenly along the length thereof. Thus, the transmission line will act as a distributed antenna allowing each fixed transceiver to cover a greater area with a relatively constant antenna coupling compared to the substantially point coupling characteristics of a conventional antenna. There will still be a problem of overlapping coverage or dead spots at the ends of adjacent transmission lines, however, a smaller number of fixed transceivers will be required to cover a given mine, tunnel or other area resulting in a smaller number of areas in which overlapping coverage or dead spots may occur. The number of transceivers required has been further reduced through the use of balanced transmission lines providing less radio frequency attenuation along their length while still allowing adequate distributed coupling of radio frequency thereto and therefrom. For example, coaxial cable having a braided outer conductor that is loosely braided to allow the desired distributed coupling has been used in the United States.
However, a network of fixed transceivers interconnected by telephone lines as described above can only provide a single channel of communication. In order to add each additional channel of communication, an additional network of relatively expensive transceivers operating at a sufficiently different frequency to avoid interference and with the requirement for power, transmission lines, control and telephone line interconnection is required.
In Europe and particularly in England, it has been thought necessary to abandon conventional radio communication techniques in seeking to provide wireless mobile-to-mobile communication in mines, tunnels and buildings. See "Systems Aspects of Leakage-Field Radio Communications" by D. J. R. Martin et al., Civil Land Mobile Conference, IEEE Conference Proceedings, November 1975.
Instead, systems have been developed in England as an entirely new technology based on the use of coaxial transmission lines modified as believed appropriate to provide coupling thereto and therefrom to synthesize the propagation of electromagnetic energy along and within the mine, tunnel, or other space. U.S. Pat. No. 3,916,311 issued to D. J. R. Martin et al., Oct. 28, 1975, is representative of the basic system according to this approach and will be more fully discussed hereinafter together with more sophisticated versions thereof. According to this new technology, specially designed and expensive coaxial transmission lines are required. In addition, two different frequencies, as well as a base station, are required to provide each channel of mobile-to-mobile communication in the mine or tunnel with the consequent need for specialized and expensive filters for equalization and complicated control circuitry as well as the different mobile transceivers for each channel of mobile-to-mobile communication.